Lindane (r-Hexachlorocyclohexane) is a widely used insecticide and may be widespread at low concentrations in the human diet. The bone marrow is a very sensitive indicator of toxicity, therefore, the possible myelotoxicity of lindane was investigated. Male B6C3F1 mice were given lindane by gavage at doses of 0, 10, 20, or 40 mg/kg B.W. for three consecutive days. Bone marrow progenitor cells numbers, hematological, and histopathological evaluations were done on days 1, 2, and 14 days following the final exposure. Lindane treated or vehicle control mice were subjected to two or four biweekly sublethal whole body irradiations (200 rads). These mice were examined at one week following the second and fourth irradiation which was 36 and 64 days following the final gavage exposure, respectively. Lindane exposure alone caused a dose-dependent decrease in erythrocyte precursors and macrophage-granulocyte progenitor cell numbers which returned to control values by two weeks. Prior exposure to lindane rendered the mice more susceptible to subsequent irradiation with a delayed recovery in bone marrow progenitor cell numbers. This effect was also reflected in marrow hypocellularity at the highest lindane dose following the fourth irradiation. This study shows that lindane has a myelotoxic effect in mice and that short-term lindane exposure can induce a residual progenitor cell damage that can be demonstrated by subsequent irradiation.